Native oysters. There are two types of sea water oyster readily available in the United Kingdom and most of Europe: rock (Crassostrea gigas) and native oysters (Ostrea edulis). Native oysters (known in France as belons) are small, flattish and circular in shape and with a brownish-green, relatively smooth shell. They are difficult to farm, take four to five years to mature and are not disease resistant. Not only are they more scarce than rock oysters, they are also more highly regarded, with firm flesh and a subtle, delicate flavour. Rock oysters are relatively abundant; the meat is held in a deep elongated cup with a flat 'lid' and the shells are rugged and warty, covered in coarse textured bumps and crevices. The flesh is less substantial than that of the natives and the flavour sharper, even metallic. When the word 'oysters' is used alone this will almost always indicate rock oysters as, if they are natives, they will invariably be described as such.